Martin Matthews, 48, along with 33-year-old Bobby Smith from Stevenage, climbed the roof of the Queen’s Gallery.

They were campaigning for the New Fathers for Justice and Stop the War on Dads group.

The duo breached security and used a ladder to scale the roof of the public art gallery at around 3pm.

Mr Matthews said he was "aware of the risks" in light of tensions following the Paris terror attacks.

Speaking to Get Surrey, he said: “It was worth the risk.

“Obviously there were a few concerns and people are going to be nervous at the moment, but Al-Qaeda were around ten years ago and Jason Hatch faced the same risk when he scaled Buckingham Palace so nothing has changed really.”

More protests will be held in the run-up to Christmas, Mr Matthews added.

Security close in as the duo perch on the roof (Image: Facebook)

The two campaigners displayed a banner reading 'I’m Harry’s dad' during the protest.

A spokesman for New Fathers for Justice explained the message.

He said: “It was an abstract reference to how children are encouraged by the alienating parent to unjustly reject the other parent.”

Parental alienation should be a "criminal offence", the spokesman added.

“The parent who, following the breakdown of a relationship, attempts to turn their child or children against the other parent should be prosecuted,” the spokesman said.

“Parental alienation is child abuse supported by the UK Family Courts and the Children and Family Court Advisory and Support Service (CAFCASS).

“Christmas is a very emotional time when you are able to see your children or grandchildren, yet more than three million children will not see one of their parents on Christmas day.”

A 'flawed' family law system is at the root of the problem, according to New Fathers for Justice.

The spokesman added: “It allows children to be exposed to a succession of mums’ fleeting boyfriends, lovers and stepfathers with whom they have no biological connection but gives no automatic right of access to their biological fathers or grandparents."